1. Field of the Invention
The present invention relates to laser drilling of sheet-like materials for printed substrates, and more particularly, to a laser drilling method and apparatus for forming through-holes in multi layered sheet-like materials.
2. Description of Related Art
Known laser drilling methods for sheet-like materials are the single pulse processing method, and the burst pulse laser train method that performs processing by multiple pulses of which each pulse has the same energy level. In these known methods, the shape of a hole is determined by a level of energy per pulse used for the processing, and when a hole with reduced taper is desired, a higher energy level per pulse is required.
In drilling a multi layered sheet-like material, however, when pulses with energy above a certain level are radiated on the sheet-like material where there are holes not yet through, pressure in the holes increases to a level greater than inter-layer adhesion force Fm. When the pressure increases in the hole before the holes have been completely through, delamination of the layers occur in the sheet-like material. As a result, clearances are produced between the layers, and when filling with electrically conductive paste this leads to seeping of the paste, into between the layers. This together with poor quality in plating results in lower reliability in electrical connection.